(1) Field of the Invention
This invention relates to the plating of platinum black on conductive substrates and, more particularly, to a method and apparatus for electrochemically plating platinum black on the outer surface of a conductive substrate while simultaneously subjecting the substrate to ultrasonic agitation.
(2) Background Art
Platinum black is a finely reduced platinum metal having a particle size of one micron or less. The particles may be as small as several hundred Angstroms in diameter. Although platinum black is pure platinum metal, a layer or film of platinum black particles appears to be black in color due to the small size of the particles.
Electrodes made of platinum and coated with platinum black are widely used as measuring electrodes. Platinum black coated electrodes, particularly thin platinum wires coated with platinum black, have been used as current collectors and oxygen reference electrodes in toxic gas sensors. One example of a toxic gas sensor is shown in U.S. Pat. No. 4,152,233, which is owned by the assignee of the present application. The platinum black coating on a platinum electrode is rather porous in nature and, as a result, increases the electrical contact area and catalytic reaction area in a gas diffusion electrode, and improves the stability of reference potentials in a reference electrode.
Presently platinum black is deposited onto conductive substrates by the electrochemical reduction of platinum(IV) complexes, usually H.sub.2 PtCl.sub.6, in a plating solution. Lead ion or the like may be added to the plating solution in order to improve the adherence of the platinum black coating to the underlying substrate. A discussion of current methods of electrochemically plating platinum black on a conductive substrate, particularly a platinum substrate, and the chemistry involved is given in an article by A. M. Feltham and M. Spiro, Platinized Platinum Electrodes, 71 Chem. Rev. 177 (1971). The platinum black coatings plated in accordance with the methods disclosed in the Feltham article, even when plated in the presence of a lead ion, are not strongly bound to the substrate. It has been found that during operation of a toxic gas sensor which includes a platinum black coated platinum electrode the platinum black coating has often completely fallen off after only several weeks of use.
It has been suggested in the prior art that annealing or sintering of plated substrates after electrodeposition will increase the strength and durability of the plated coating. However, annealing is an additional, time consuming step which also leads to a reduction in the electrochemical surface area of a platinum black coating on an electrode.
The use of ultrasonic agitation in connection with plating of copper is known in the art. S. I. Uspenskii et al., The Effect of Ultrasonics on Cu Electrodeposition, 2 Elektrokhimiya 243 (1966). Ultrasonic is a term given to the physical vibration of particles at frequencies above the auditory limit of the human ear or above approximately 16,000 cycles per second. In practice, the ultrasonic frequency spectrum includes frequencies in the range of 16,000 cycles per second to several million cycles per second or higher. Ultrasonic agitation has been used in conjunction with the electroplating of cadmium, copper, silver, chromium and nickel, S. R. Rich, Improvement in Electroplating Due to Ultrasonics, 42 Plating 1407 (1955), and for the electroplating of smooth, poreless platinum on copper, F. I. Kukoz et al., Formation of Nonporous Galvanic Deposits of Platinum in an Ultrasonic Field, 39 Zh. Prikl. Khim 705 (1966). However, the prior art has in no way taught or suggested that ultrasonic agitation may be used in conjunction with the electrochemical plating of platinum black on a conductive substrate, particularly a platinum substrate.
It is an object of the present invention to plate, on a conductive substrate, a platinum black coating which is highly adherent to the substrate and will remain on the substrate during use as an electrode or the like.
A further object of the present invention is to plate a durable platinum black coating on a conductive substrate without increasing the time needed to carry out the plating process and without decreasing the electrochemical surface area of the coating.